Graphene poly(vinylidene fluoride) composites

Graphene/poly(vinylidene fluoride) composites with high dielectric constant and low percolation threshold This article has been downloaded from IOPscience. Please scroll down to see the full text article. 2012 Nanotechnology 23 365702 (http://iopscience.iop.org/0957-4484/23/36/365702) Download details: IP Address: 222.205.2.170 The article was downloaded on 15/09/2012 at 07:44 Please note that terms and conditions apply. View the table of contents for this issue, or go to the journal homepage for more Home Search Collections Journals About Contact us My IOPscience IOP PUBLISHING NANOTECHNOLOGY Nanotechnology 23 (2012) 365702 (8pp) doi:10.1088/0957-4484/23/36/365702 Graphene/poly(vinylidene fluoride) composites with high dielectric constant and low percolation threshold Ping Fan, Lei Wang, Jintao Yang, Feng Chen and Mingqiang Zhong College of Chemical Engineering and Material, Zhejiang University of Technology, Hangzhou, 310014, People’s Republic of China E-mail: zhongmingqiang@hotmail.com Received 6 June 2012, in final form 18 July 2012 Published 21 August 2012 Online at stacks.iop.org/Nano/23/365702 Abstract In aiming to obtain highly flexible polymer composites with high dielectric performance, graphene/poly(vinylidene fluoride) (PVDF) composites with a multi-layered structure were proposed and prepared. Graphene sheets were prepared by reducing graphene oxide using phenylhydrazine, which could effectively alleviate aggregation of the graphene sheets. A two-step method, including solution casting and compression molding, was employed to fabricate the graphene/PVDF composites. The composites showed an alternative multi-layered structure of graphene sheets and PVDF. Due to their unique structure, the composites had an extremely low percolation threshold (0.0018 volume fraction of graphene), which was the lowest percolation threshold ever reported among PVDF-based polymer composites. A high dielectric constant of more than 340 at 100 Hz could be obtained within the vicinity of the percolation threshold when the graphene volume fraction was 0.00177. Above the percolation threshold, the dielectric constant continued to increase and a maximum value of as high as 7940 at 100 Hz was observed when the graphene volume fraction was 0.0177. (Some figures may appear in colour only in the online journal) 1. Introduction Poly(vinylidene fluoride) (PVDF) and its copolymers have attracted much attention for their variety of applications in electromechanical systems [1–4]. However, a high dielectric constant is highly desirable when they are employed as functional materials, such as high-storage capacitors and electrostriction system for artificial muscles. Various methods have been developed to increase the dielectric constant of PVDF composites. The traditional approach is to disperse high dielectric constant ceramic powder in PVDF to prepare 0–3 type composites. To achieve sufficiently high dielectric constant, a very high filler content (usually over 50 vol%) is in general necessary. Such high content of ceramic powders significantly degrades the mechanical performance of the composites. To address this problem, conductive-filler/polymer com- posites with percolation threshold have been proposed [5]. Because a small volume fraction of conductive filler is able to achieve high dielectric constant, the flexibility of the polymer matrix is preserved [6–15]. Metal particles [6–8], acetylene black [9], carbon fiber [10], carbon nanotubes [11–14], and graphite nanoplates [15] have been used as conductive fillers. Graphene, a flat monolayer of carbon atoms tightly packed into a two-dimensional honeycomb lattice, is emerging as a rising star in the field of materials science because of its unique electronic, thermal, and mechanical properties [16–18]. Compared to carbon nanotubes, which can be regarded as rolled-up graphene sheets, graphene can be obtained from cheap graphite by simple chemical treatments. Moreover, graphenes have many interesting transport properties, such as the quantum Hall effect and quantum tunneling effect [19–21]. Therefore, it is generally accepted that graphene will be superior to carbon nanotubes in many applications. There have been reports on graphene/PVDF composites with interesting dielectric 10957-4484/12/365702+08$33.00 c© 2012 IOP Publishing Ltd Printed in the UK & the USA Nanotechnology 23 (2012) 365702 P Fan et al properties. Cui et al [22] prepared a graphene/PVDF composite with percolation threshold of 4.08 vol%. A maximum dielectric constant of 2080 at 1000 Hz was achieved for the composite with 12.5 vol% graphene. Song et al [23] prepared graphene/PVDF composites by ultrasonic processing and mechanical mixing. In both cases, no effort has been made to control the distribution of graphene and the morphology of the composite. Therefore, graphene sheets were randomly dispersed in the PVDF matrix, which easily form conductive network so that it is difficult to achieve a high dielectric constant. Since graphene has an excellent electrical conductivity and a flake shape with high aspect ratio, it is possible to form a microcapacitor in the composite if two graphene sheets have a compact parallel structure, isolated by a thin layer of the polymer. This is because it has been shown that the dielectric constant of polymer composites can be greatly increased by having numerous microcapacitors [24–27]. To obtain such a structure, special attention must be paid. The first problem is the aggregation of the graphene sheet. It is well known that pristine graphene consists of carbon atom layers packed densely in a honeycomb crystal lattice without any polar group. Thus, pristine graphene sheets are not intercalatable by large species, such as polymer chains, because they have a pronounced tendency to agglomerate in a polymer matrix. The second problem is the random distribution of the graphene sheets in the composite. In order to solve the first problem, graphenes were prepared by reducing graphene oxide (GO) with phenylhy- drazine. Fourier transform infrared (FTIR) spectroscopy and x-ray photoelectron spectroscopy (XPS) were used to confirm whether the phenyl group of phenylhydrazine was introduced on the graphene sheets. In order to solve the second problem, a mixture solution of PVDF and graphene sheets were cast into films and then several solution-cast films were stacked layer by layer and hot-pressed into graphene/PVDF composites. Morphology of the composite was examined by using a scanning electron microscope (SEM). Broadband dielectric spectroscopy was applied to study the electrical conductivity and dielectric constant of the graphene/PVDF composites over a wide frequency range (10–106 Hz). 2. Experimental details 2.1. Materials Natural flake graphite used in this study was supplied by Guangli Graphite Co., Ltd, Qingdao, China. N,N- dimethylacetamide (DMAc), 98% sulfuric acid (H2SO4), 30% hydrogen peroxide (H2O2), sodium nitrate (NaNO3) and potassium permanganate (KMnO4) were purchased from Shuanglin Chemical Reagent Factory of Hangzhou, China and were used as received without purification. PVDF (FR901) was purchased from Shanghai 3F New Material Co., Ltd. 2.2. Synthesis of GO GO was synthesized from purified natural flake graphite by a modified Hummers method [28, 29]. Briefly, graphite powder (5 g), NaNO3 (3.75 g) and concentrated H2SO4 (200 ml) were added into the 1000 ml flask and stirred uniformly in an ice bath. Then, KMnO4 (40 g) was gradually added with stirring and cooling in order to keep the temperature below 20 ◦C. The mixture was then stirred at room temperature for about 24 h until a viscous fluid was obtained. Then, 5 wt% of dilute H2SO4 (500 ml) was slowly added and the temperature was controlled to be lower than 100 ◦C. After stirring for 2 h, the reaction was terminated by adding 30% H2O2 solution (26.97 ml). The mixture was left overnight. Graphite oxide particles, settled at the bottom, were separated from the excess liquid by decantation, followed by centrifugation. Then, it was washed with a mixture aqueous solution (the volume ratio of water, H2O2 and H2SO4 is equal to 1:0.23:0.26) and deionized water. Graphite oxide was obtained after drying. 100 mg graphite oxide was dispersed in 100 ml of water to create a yellow-brown dispersion, and the exfoliation of graphite oxide to GO was achieved by sonication with a cylindrical tip for 1 h. 2.3. Reduction of GO In this paper, graphenes were obtained by reducing the GO sheets using phenylhydrazine as a reducing agent. For comparison, graphenes were also prepared by reducing the GO using hydrazine hydrate as a reducing agent. The graphene sheets reduced by hydrazine hydrate and phenylhydrazine were marked with G1 or G2 respectively. In a typical synthesis procedure, about 100 mg GO was dispersed in 100 ml deionized water. The dispersion was ultrasonicated until it became clear, without visible particulate matter. Subsequently, hydrazine hydrate (1 ml, 32.1 mmol) was added to the solution to prepare G1. After heating at 100 ◦C for 24 h, the mixture turned from yellowish brown to black. The mixture was then cooled, filtered and washed several times with deionized water. The product was dried at 50 ◦C under vacuum overnight to obtain G1. To synthesize G2, phenylhydrazine (1 ml) was added to the GO solution, which was then kept for 24 h at room temperature. The reduced GO, precipitated as a black solid, was isolated by filtration. Then it was washed several times with ethanol and DMAc, and finally dried at 50 ◦C under vacuum overnight to obtain G2. 2.4. Preparation of graphene/PVDF composites The G2 were ultrasonically dispersed in DMAc for 3 h in order to form a stable suspension. At the same time, PVDF powder was also dissolved in DMAc. Then, both suspensions were mixed. The mixture was subjected to ultrasonic dispersion for another 3 h and cast into film. In order to completely remove DMAc, the mixture was first dried at 60 ◦C for 24 h and then dried in vacuum oven at 80 ◦C for 12 h. Films formed after evaporation of DMAc were further stacked layer by layer and hot-pressed at about 200 ◦C and 15 MPa into disk-shaped samples of about 9 mm in diameter and 5 mm in thickness. 2 Nanotechnology 23 (2012) 365702 P Fan et al Figure 1. FTIR of GO, G1 and G2. 2.5. Characterizations JEOL JEM-1230 transmission electron microscopy (TEM) at an acceleration voltage of 80 kV was used to observe the morphology of the graphene sheets. The samples were prepared by dispersing the graphene sheets in methanol and drop coating onto a copper grid. FTIR spectra analysis was performed by using a Nicolet 6700 FTIR spectrometer (Thermo Co., USA). All infrared spectra were scanned from 4000 to 400 cm−1 with a resolution of 4 cm−1. XPS was recorded on a Kratos Axis Ultra-DLD system (Shimadzu Co., Ltd, Hong Kong) with Al Kα ray (1486.6 eV) and the C1 s peaks were fitting according to a Gaussian distribution. The morphologies of the G2/PVDF composite were examined using a Hitachi S-4700 SEM. The samples were fractured in liquid nitrogen and coated by gold. X-ray diffraction (XRD) patterns were performed at room temperature using an X’Pert PRO diffractometer (PANalytical, Holland) with Cu radiation (36 kV, 30 mA). All XRD data were collected in the range of 2θ = 10◦–80◦ at a step of 0.02◦. The dielectric properties of the composites were characterized by means of a broadband dielectric spectrometer (Turnkey Concept 80, Novocontrol Tech. GmbH & Co. KG, Hundsangen, Germany) at room temperature using the gold-pasted sample. The value of AC voltage applied to the samples during measurements is 1 V and the samples were polished by fine sandpaper before gold sputtering in order to diminish the contact resistance. 3. Results and discussion FTIR results of GO, G1 and G2 are shown in figure 1. After reduction, the absorption peaks at about 1575 and 1722 cm−1, which correspond to the carbonyl vibration peak, disappear in the FTIR spectra of both G1 and G2. Furthermore, compared to G1, which does not show obvious absorption peaks in its FTIR spectrum, absorption peaks corresponding to the structure of styrene appear in the FTIR spectrum of G2. The bands at 2917, 2847, 1620 and 670 cm−1 correspond to C–H unsymmetrical stretch, C–H symmetrical stretch and the aromatic ring vibration, which demonstrates that the phenyl group of phenylhydrazine was covalently bonded onto the G2 sheets. Figure 2. XPS wide spectrum of the GO, G1 and G2. Figures 2 and 3 are the full XPS spectra and narrow scan spectra of C 1s of the GO, G1 and G2. As shown in figure 2, after reduction, the level of carbon increases while the level of oxygen decreases. The C1s signal of GO was peak-fitted with four components which represent the carbons atoms in different functional groups [30]: (1) aliphatic hydrocarbon (C–C/C–H, at a binding energy of 284.8 eV), (2) hydroxyl or epoxide carbon (C–O at 286.0 eV), (3) carbonyl carbon (C=O at 287.1 eV) and (4) carboxyl carbon (−COO at 288.9 eV). The C 1s spectra of G1 and G2 also exhibit peaks corresponding to the functional groups with oxygen, but their intensities are much lower than those in GO, indicating some oxygen functional groups were removed during the reduction process. For preparing composites by the solution blending method, the distribution of graphene sheets in the PVDF matrix is largely determined by their dispersion state in the solvent. Therefore, the dispersion stability of G1 and G2 in DMAc were investigated. As shown in figure 4, G1 precipitates rapidly while G2 has good dispersion stability. The good dispersion stability of the G2 in DMAc might be attributed to the steric effect of the phenyl groups of phenylhydrazine that are covalently bonded with the graphene sheets. Therefore, G2 was chosen to prepare graphene/PVDF composite. Typical SEM and TEM images are shown in figure 5. It can be observed that the G2 consists of randomly aggregated, thin and crumpled sheets closely associated with each other by forming a disordered solid flake. The thickness of the sheets is in the range of 2–6 nm (figure 5(a)). The morphology of graphene is more obvious in the TEM image. SEM images of the composites with G2 of 0.001 77 and 0.0177 volume fractions are shown in figure 6. The graphene sheets are homogeneously dispersed in the PVDF matrix without serious aggregation. Most graphene sheets are perpendicular to the fracture surface and parallel to one another. However, due to the low content of graphene, the distance between neighboring sheets is large, so that only a small number of microcapacitors are formed. As the graphene content is increased, an obvious multi-layered structure with graphene sheets intercalated by PVDF layers is observed 3 Nanotechnology 23 (2012) 365702 P Fan et al Figure 3. XPS carbon 1s core-level spectra of the GO, G1 and G2. Figure 4. Photo images of the G1 and G2 dispersed in DMAc. Figure 5. SEM image (a) and TEM image (b) of the G2. 4 Nanotechnology 23 (2012) 365702 P Fan et al Figure 6. SEM micrographs of the graphene/PVDF composite containing graphene volume fractions of 0.001 77 ((a), (b)) and 0.0177 ((c), (d)). Figure 7. XRD patterns of the pure PVDF and the graphene/PVDF composites with different graphene contents. (figures 6(c) and (d)). After the solution was cast and the solvent evaporated, the graphene sheets with large aspect ratios tended to preferentially orient in the plane of the film. The hot-press process further strengthened the preferred orientation of graphene sheets. It has been widely reported that PVDF shows highly complicated crystalline structures and exhibits at least five possible types of crystal phase (α, β, γ , δ and ε). The presence of polar β-phase is of crucial importance to show piezoelectricity. As shown in figure 7, with increasing content of graphene, the amount of the β-phase (2θ ≈ 20◦) increases slightly and that of the α-phase (2θ ≈ 18◦) decreases slightly. Although it is accepted that the increase of the β-phase is beneficial for piezoelectric properties, the slight increase in Figure 8. AC conductivity of graphene/PVDF composites as a function of graphene volume fraction, measured at 100 Hz and room temperature. The insets show the best fit of conductivity to equation (1). the β-phase in this case will contribute little to the dielectric constant. In inorganic–organic conducting-polymer-based com- posites, the critical volume fraction at the percolation threshold, fc, is a key parameter when studying their electrical properties [3, 26]. Near the percolation threshold, the elec- trical conductivity and dielectric constant of the composites increase by several orders of magnitude. Figure 8 shows the alternating-current (ac) conductivity of the graphene/PVDF composites as a function of graphene volume fraction (fgraphene), measured at room temperature and 100 Hz. The 5 Nanotechnology 23 (2012) 365702 P Fan et al conductivity can be further analyzed with the critical graphene content fc by the power laws [24, 25, 31, 32] , σeff ∝ σi(fc − fgraphene)−q for fc > fgraphene (1a) σeff ∝ σi(fgraphene − fc)t for fc < fgraphene (1b) where σeff is the effective conductivity of the composites, σi is the conductivity of the insulating PVDF, fgraphene is the graphene volume fraction, fc is the critical volume fraction at the percolation threshold, q is the critical exponent in the insulating PVDF, and t is the conductivity exponent. The best linear fit of the conductivity data to log–log plots of the power laws for equation (1) gives fc = 0.0018 and t = 3.81 (the inset in figure 8). To the best of our knowledge, this percolation threshold is the lowest, compared with those reported previously. The critical exponent in the conducting region, t = 3.81, is higher than the universal ones (tun ≈ 1.6–2). Similar values have also been reported in the multiwall carbon nanotube (MWCN)/PVDF composites [14] and MWCN/polycarbonate composites [31]. The t values might be related to the micro-structural properties (i.e. filler size, shape etc) of the conductive-filler/PVDF composites. Figure 9 shows dielectric constant and dielectric loss of the composites. The addition of graphene led to composites with remarkably increased dielectric constant. The dielectric constant of the sample with fgraphene = 0.001 77 is 340, which is about 30 times higher than that of pure PVDF (about 10). A even higher dielectric constant of 7940 can be obtained for the graphene/PVDF composite with fgraphene = 0.0177, which is approximately three orders of magnitude higher than the value of pure PVDF. When the graphene content is near the percolation threshold, the dielectric constant can be expressed by the percolation-theory power law [6, 24], εeff ∝ εi(fc − fgraphene)−s, for fgraphene < fc (2) where εeff is effective dielectric constant of the composites and s is critical exponent. From the best fitting, we got fc = 0.0018 and s = 1.09. The critical exponent s agrees well with the universal one (s ≈ 1) [24]. We believe that the great increase of dielectric constant of the graphene/PVDF composites can be attributed to the formation of the multi-layered structure. The variation of dielectric constant versus fgraphene can be explained in light of the microcapacitor model. Namely, a pair of neighboring graphenes is regarded as a microcapacitor, with the graphenes as the two electrodes and a very thin PVDF layer in between as dielectric. A network of these microcapacitors expands between two virtual electrodes with increasing graphene content. Each microcapacitor contributes an abnormally large capacitance. The evolution of the dielectric constant of the graphene/ PVDF composites can be divided into three stages (A, B, and C). Initially, when a small amount of graphene is incorporated, the graphene sheets are isolated from one another and only a small number